Note to producers: If you want your torn-from-true-life tale of a violent ex-criminal turned spiritually awakened savior of children to be taken seriously, it might not be the best idea to call your movie “Machine Gun Preacher,” cast Gerard Butler in the title role of a self-described Pennsylvania hillbilly and promote the film with a poster featuring a buff, bearded Butler in T-shirt, a machine gun in one hand, and a sad-faced African child being held out of harm’s way by the other.

True, the real-life Sam Childers identifies so closely to the Machine Gun Preacher name – it’s practically become a brand – that he’s tattooed it on his right arm. But, even this ex-biker with the slicked-back hair, slow drawl and Sam Elliott-like handlebar mustache had the good sense to title his 2009 memoir “Another Man’s War,” rather than something that evokes equal parts “Hobo with a Shotgun” and grindhouse fare like “Machete.” Still, maybe the only way this material could have succeeded onscreen would have been as a full-blown exploitation picture, or possibly as a straightforward documentary. (In fact, Childers is producing just that.) But, as directed by Marc Forster, this is an over-earnest slog through born-again cliches which only comes alive – for better or worse – when Jason Keller’s script embraces the over-the-top, Rambo-saves-Sudan heroics of the film’s second half.

Before Childers finds his calling, he has to get out of prison, immediately have sex with his wife, Lynn (Michelle Monaghan of “Gone Baby Gone”), become violently upset with her for giving up stripping in deference to God, reconnect with his (fictional) best friend Donnie (Michael Shannon) so the drugged-up duo can rob some dealers at gunpoint and nearly kill a crazed, knife-wielding hitchhiker.

Furiously scrubbing his hands of that thumb-wagger’s blood as his saintly wife looks on, Childers realizes his hands have been dirtied long enough, and his path toward redemption starts by replacing that blood with calluses in his newfound line of work as a carpenter. Soon (at least in the compressed timeline of the film; the real Childers’ awakening occurred over the course of four years), Childer’s is building a church and beginning his own ministry.

A five-week-long religious mission to East Africa to help rebuild homes destroyed by civil war turns into a years-long combat mission after Childers melodramatically witnesses an orphan lose his limbs to a land mine. Sure, he’ll build and establish an orphanage for these children of war, even if saving them means he’ll have to take up arms against warlord Joseph Kony and his Lord’s Resistance Army, a renegade militia that forces youngsters to become soldiers even before they reach their teens.

Even though the film continues the “white witness to black problems” theme most recently exploited in “The Help,” Childers does far more than pick up a pen, becoming the titular machine gun-wielding preacher. Mythologizing a man who fights fire with firepower may not appeal to God-loving moviegoers as much as God-fearing ones, but the movie’s just too damned confused to try and make the distinction. Is Childers a rebel, or a mercenary? Forster can’t seem to make up his mind, and the director’s hand is just as heavy as it was when guiding his deeply flawed adaptation of “The Kite Runner.”

Meanwhile, Childers’ wife and their teenage daughter Paige (Madeline Carroll of “Mr. Popper’s Penguins”) begin to resent the amount of time he’s away, and their scenes back in the U.S. take on an overwrought air that wouldn’t feel the least bit out of place on a nighttime soap, what with Paige screaming howlers like “you love them black babies more than you love me!” And pity poor Shannon, an Oscar nominee for “Revolutionary Road,” who can’t do much with his cartoonish role as a doomed dope addict whose fate serves as a wake-up call for our flawed hero right when he’s hit his spiritual nadir.

But, if that somber moment briefly spoils the gung-ho mood, there’s still that orphanage full of sometimes maimed, always dewy-eyed, nameless kids in Sudan to tug at your heartstrings, before Butler blasts back into action for them.

Forster is nothing if not well-intentioned, but as a director, he’s never had much of an eye or ear for subtlety or consistent tone, nor can he resist the urge to wield his camera like a blunt instrument, hammering you over the head with cheap sentiment when simply observing would suffice.